As one of skill in the art knows, a free piston engine (FPE) is characterized in that it does not have a mechanical linkage (for example, a crankshaft) by means of which reciprocating motion of the piston or pistons is directly converted to motion of another mechanical element or elements. (Free piston engines can be of a single piston or dual piston layout.) In a dual piston layout, there are separate combustion chambers and fuel injection for each piston. The pistons are attached to each other by a rod so that they move simultaneously. A common way of attaining energy from a FPE is to mount an electrical generator to the interconnecting rod.
Another known engine is of the so called HCCI type combustion. When a homogeneous, diluted (air or residual gas fraction) fuel/air mixture is compressed in an engine at such a high temperature that it auto-ignites, the engine is said to operate on homogeneous charge compression ignition (HCCI).
U.S. Pat. No. 6,199,519, for example, discloses a two-stroke, free piston engine with a linear electrical alternator/generator. The engine is driven in an oscillating mode with constant frequency. Fuel and air are introduced in a two-stroke cycle fashion without using valves. The engine operates in an HCCI (autoignition) mode and will shut off when not needed and run at essentially constant velocity and power when on. Regulation of the coil current can be used to regulate the piston velocity as to change the compression ratio if the fuel composition is changed.
The engine/generator is particularly intended for charging batteries in hybrid automobile applications.
To start the engine, the batteries are used to operate the linear alternator as a linear motor. The piston is oscillated in the cylinder, building to a higher compression each cycle until sufficient compression is developed for autoignition and at which time fuel is introduced to the engine and self powered operation will ensue.
WO 01/45977 describes a device including a free piston engine, an electromagnetic energy transducer, a combustion system and a control unit. The combustion system can include one or two active combustion chambers and operates in two strokes, and alternatively four strokes. The device makes it possible to determine the instantaneous position of the piston. By controlling the electrical power to and from the electromagnetic energy transducer, and in combination with spark plugs that are also controlled by the control unit, it is claimed to be possible to displace the piston to a desired position when a combustion is to be initiated by the spark plugs. It is also proposed to control and vary the compression depending on the instantaneous load on the combustion engine (e.g., by increasing the compression to obtain a higher power output). HCCI is mentioned as a possible combustion principle, but there is nothing disclosed on how such a combustion could be controlled. Further, different storing devices for storing at least a part of the combustion energy such as capacitors, batteries and flywheels are discussed. Their use is, however, scarcely described.
Although the existing free piston engine energy converters do convert chemical energy to electric energy in a fairly good way via combustion, they are driven in a constant load situation and therefore these solutions require large energy storage facilities such as battery packs. This type of equipment is expensive and therefore lowers the efficiency of the system.